At first impression, Dokuga can come off as completely obsessed with the Boss. He is perhaps Kai's most zealous follower and hardly a scene goes by without him thinking of Kai. But, on closer inspection, their relationship is much more nuanced than that.
Life Debt
Dokuga feels that he owes Kai a life debt for killing the magic users who left him and his friends to die in Hole. Kai wasn't really trying to save them, and frankly it's a miracle he didn't kill them too. Still, to a bunch of street kids forsaken by the world, he was a beacon of hope, and they followed him.

Dokuga dedicated his life to being useful to his beloved Boss, and how seriously he took that role is shown when two of his friends' lives were on the line, and the other CrossEyes Officers were ready to pawn in Kai's daggers to get money for healing smoke. Dokuga angrily refused, and instead opted for a much riskier way.
Civil Rights
Civil Rights
This wasn't because he loved Kai more than his friends. It was because Kai was a symbol for something much bigger than Dokuga and his friends, and he felt responsible for protecting that.

The sorcerer world ran on principles of Social Darwinism. There were no laws to speak of. Murder, torture, maiming, and theft went unpunished save for vigilante justice. Might made right and only the fittest survived.

Though sorcerers liked to test their magic on the humans of Hole, they treated members of their own society no different. Those that couldn't use magic, those whose magic wasn't powerful enough, or those who had useless kinds of magic were subjected to bullying and abuse. No matter how hard they tried, they were unable to get decent jobs and were often driven to a life of crime. Meanwhile, those fortunate enough to be born with strong magic could effortlessly live a life of luxury by selling their smoke and services.

That is why, to the helpless and the oppressed, the CrossEyes Boss was a long-awaited Messiah. He embraced social outcasts and struck fear into the magic user elite. He gave the world black powder, which amplified magic and allowed less abled people to lead more normal lives. He taught his followers how to defend themselves by disarming magical abilities.

But what the masses interpreted as Kai standing up for the underdog was just him acting in his own self-interest. He was collecting magical powers.
And Dokuga was more to blame than anyone for perpetuating the myth of the Boss as an altruistic savior. As the Boss' right hand man, Dokuga was the keeper of his most alarming secrets. He knew, better than anyone, what Kai was really like. It made no difference to Kai if the powers he sought were in the heads of comrades or enemies. He was an equal-opportunity butcher.

This greatly disturbed Dokuga, though he didn't show it, or tell even his closest friends. He was an expert at masking his emotions, and felt this was necessary to maintain their morale. When he was finally forced to share what he knew with Tetsujo, the latter's reaction was predictably negative.
But there was also a more practical dimension of self-preservation to Dokuga's motives. He was aware that they owed everything they had to Kai and, if it wasn't for his protection, they'd all be dead.

Not in some hypothetical sense of what could've been if Kai hadn't passed by that one rainy night, but in the immediate sense that they would be dirt poor and hunted by magic users for the trouble they've caused.
It's significant to note that, even given black powder and magic-disarming techniques, the CrossEyes and En's forces were not evenly matched. The Officers were quite humble about their abilities and knew they couldn't fight the powerhouses of the sorcerer world without Kai. They scattered like cockroaches if they saw one and confrontation was avoidable.

That's why, whenever the Boss was in danger, there was the constant tension between "there goes my meal ticket" and "oh no, civil rights!" And the CrossEyes were, essentially, civil rights activists. Freedom fighters to the oppressed, terrorists to the oppressors, but the status quo left them with no choice. They pursued their cause through the only channel available.
Though Kai's methods were brutal and cruel, good still came out of them. Even if it was unintentional, indirect, or accidental, it still helped, and was a lot better than nothing.

It was very hard for Dokuga, being very sensitive and idealistic by nature, to accept that Kai wasn't the person he assumed him to be. But that didn't change that Kai saved him and his friends, took them under his wing, made them feel worthwhile and filled their lives with purpose. It didn't change that Kai was an inspiration to the weak and had already done so much to shift the balance of power in society.

Dokuga desperately wanted to believe that there was more to it than Kai using them for purely selfish gain. That there was a reason why he shared with them the tools to make the world a better place for others like them. That some part of him cared about the CrossEyes and their cause. And that, even if some lives had to be sacrificed for Kai's ambition, such was the price of their revolution.

At the very least, he may have hoped that staying at Kai's side gave him the opportunity to have some influence over his decisions.
Send Away Send Away
This is probably why he had the misguided notion that he can "talk" to the Boss.

After it was discovered that a CrossEyes member they had befriended actually had strong magical powers, the other Officers rejoiced and wanted her to be the Boss' bodyguard, but Dokuga and Tetsujo knew her life was in danger. This time it was personal, and Dokuga wasn't about to let her die. He was willing to put himself on the line by actively sabotaging what he knew Kai would want.

He planned to send her away to buy some time for him to reason with Kai about the wisdom of killing fellow CrossEyes. Of course, this was based on the naive assumption that somewhere, deep down, Kai had a conscience, or some capacity for sympathy, or a soft spot for Dokuga's tireless devotion. Or, failing all those, that there at least existed a coherent personality that could be reasoned with.
But there wasn't.

Natsuki was a girl who desired nothing more than to be useful to Kai. She had helped him, healed him, and talked to him. But none of that mattered. He killed her anyway, using her love for him against her.

Kai was literally a monster. Just a killbot created by Ai to gather ingredients for his ideal self. He didn't care about anything besides this. He couldn't even care that that Dokuga tried to send his prey away. He was completely hollow, and any semblance of a person that might've been there was long dead.
Natsuki pick up
Dokuga had suffered in silence for so long. It hurt to think that he bound himself to a cold-blooded tyrant, and that he had built his life around an illusion. As second in command, and especially during Kai's absence, he thought he had to be strong for everybody, so he acted like nothing was wrong. He put on a stoic face and carried all the pain alone.

But Kai never claimed to be any of the things the CrossEyes attributed to him. It was their own wishful thinking that led them to see their Boss that way, and Dokuga never bothered to correct them. He chose to guard the sanctity of Kai's reputation because he thought their cause would fall apart otherwise. It was a tough call to make. One that knowingly endangered many others.

Though he was deeply conflicted about Kai's methods, he accepted the world as a merciless place that only listened to violence, and trusted that Kai would ultimately work towards the greater good. But what good was it? What kind of world was he fighting for if he couldn't even protect his friends?
What am I supposed to do?
Dokuga was the one with the most faith in the Boss, but his faith wasn't blind. It was based on firsthand knowledge of what Kai was capable of, both good and bad. He saw it as a balancing act of costs and benefits, the lesser of possible evils when viewed from the bigger picture - along with a healthy dose of fear. Fear of Kai, and fear of losing Kai.

Certainly Dokuga had motives that could be called selfish or cowardly, but instead of indulging them through corruption and greed, he used them as the foundation of his compassion for all who struggled like he did. That is why it's a great irony that it was Dokuga's heightened kindness and sincere desire for a better world that convinced him to tolerate as much from Kai as he did.

The other Officers were much less subtle. Much as he wanted to be honest with them, Dokuga knew it would be a very bad idea to tell them about Natsuki. They would want answers right away, and be as appalled at Dokuga's apparent complacency as Tetsujo was. Their brashness would only annoy Kai, or worse. But Dokuga was no less adamant in his own way. He never stopped thinking for himself and was always questioning. He had the tact to work around Kai, and even against him, without getting himself killed.

As the one with the most knowledge, he was also the one with the most doubts, since he had a better understanding of how much he didn't know. He tried not to think about it, but he let it spiral out of his control, and now the powerlessness he felt about his own role and Kai's actions was nauseating. Between wanting to protect himself, his Boss, his friends, and salvaging what he could of the ideals of the CrossEyes, he was being pulled in too many directions.
I can no longer
Despite these festering feelings of uncertainty, it never crossed his mind to outright betray Kai. He still wanted to talk to him before deciding what he would do and what he would tell the others. This conflicted mental web of trust, conviction, idealism, obligation, regret, fear, doubt, and hope makes Dokuga a very "human" character, complete with an irrational side prone to self-delusion.

Until he could confront Kai and straighten things out, Dokuga remained fiercely protective of the Boss and his interests, no matter how crazy. His loyalty to the Boss ran very deep, his emotional investment too strong, and his sense of duty was neigh unshakable. He continued to stain his hands with blood, doing all the dirty work that eroded his soul, and leading others to do the same. He followed Kai's orders to massacre hundreds of innocent magic users a day to find a needle in a haystack.

It was getting harder and harder to trust the Boss' intentions when everybody was stretched to the limits of their sanity, and the other Officers began to question Kai's motives. Dokuga just kept quiet. He wanted to discuss this with Kai on his own terms. After confirming Natsuki's death, he went looking for Kai in his chambers, but it was too late.
Why are you here?
He never got his chance. Kai's time among them had run its course and he fused into a wholly new and suspiciously helpless kind of being.

Dokuga wasn't about to abandon his Boss in such a state. He took the initiative to help this creature up and try to figure out what it wanted. And besides, if he did give up, he'd never learn what this was all for, and everyone would've killed and died for nothing.

Nobody knew what was happening. There was no point in blame, anger, or regret. The only thing that could be done was to stay at the Boss' side and hope that he knew what he was doing.
Darth Kai
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DISCLAIMER: all story, art, characters are copyright Q Hayashida. This is a fansite. No copyright infringement is intended. Translations by Gantz Waiting Room, VIZ Media, and sometimes myself. Dorohedoro is published in Ikki Magazine.